1. Technical Field
The present invention pertains to a planar inverted F antenna, and relates to an antenna used, for example, in electronic communication equipment such as a mobile phone and the like.
2. Background Art
Recently, the planar inverted F antenna has been used as a high-performance antenna which can be built in small-sized electronic communication equipment such as a wrist watch, a portable terminal, a sensor and the like, and various proposals have been made as indicated in Patent Documents 1 and 2.
FIG. 25 shows a basic structure of the inverted F antenna.
The planar inverted F antenna is configured by a ground conductive plate 100 which has been grounded, a main conductive plate 300 which functions as an excitation conductive plate to be arranged almost in parallel with the ground conductive plate 100 with a length of (¼)λ or in the neighborhood thereof relative to a wavelength λ, a short circuit plate 200 for short-circuiting the main conductive plate 300 and the ground conductive plate 100, and a feeding pin 410 which has been connected to the main conductive plate at a position apart from the short circuit plate 200 by a prescribed distance s.
A feeding line to the main conductive plate 300 is configured such that a through-hole 110 is formed in the ground conductive plate 100 so as to supply power from below the ground conductive plate 100 side through the through-hole 110, thereby minimizing the influence on antenna characteristics.
Then, a central conductor of a coaxial line 400 is connected to the main conductive plate 300 as the feeding pin 410, while an external conductor 420 is connected around the through-hole 110 in the ground conductive plate 100.
In such a planar inverted F antenna, it is necessary to set the feeding impedance of the main conductive plate 300 to 50Ω from its relationship with a circuit to which the antenna is to be connected, and therefore a point which is at the prescribed distance s from the short circuit plate 200 is set as a feeding point and the feeding pin 410 is connected to this feeding point.
This prescribed distance s is determined in accordance with various conditions such as a distance between the ground conductive plate 100 and the main conductive plate 300, a dielectric constant ∈ between them and the like and is reduced as the planar inverted F antenna is miniaturized.
At a frequency which is generally used in the mobile phone or the like, this prescribed distance s is not more than 10 mm in many cases and is not more than 1 mm in some cases depending on the conditions.
Then, the prescribed distance s relative to the feeding point is a value which is strictly determined and even a slight shift (for example, a shift of 0.1 mm) will result in a shift in feeding impedance from 50Ω. A power loss is induced by this mismatching and it becomes impossible to obtain desired antenna characteristics.
Thus, in the conventional planar inverted F antenna, it was necessary to accurately attach the feeding pin 410 to the feeding point.
Then, since higher positional precision has been demanded in a narrow range of not more than 10 mm with respect to the attachment position of the feeding pin 410, the attachment procedure has been troublesome.
In addition, in the conventional planar inverted F antenna, while the connection spot of the feeding pin 410 is situated in the vicinity of the short circuit plate 200, the position of radiation by the antenna is situated on the open end side opposite to the short circuit plate 200.
Since the feeding position and the radiation position are situated on the opposite sides as mentioned above, if the feeding position of this antenna is arranged on the end side of the electronic equipment, connection of the feeding pin 410 will become easy, but the radiation position will get into the device. Therefore, it sometimes occurred that the antenna performance is deteriorated under the influence of an electric circuit or, in case of the mobile phone, under the influence of the hand of a person who grips it.
To the contrary, if the radiation position is arranged on the end side of the electronic equipment by prioritizing the antenna performance, the feeding position will be within the device, and therefore, such a problem occurred that the connection of the feeding pin 410 becomes not easy.